Current Control Circuit for Power Converter with Sense Transistor
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Solution Overview
Problem
Current current sensing techniques in power converters face challenges such as additional conduction losses and inaccuracy due to varying on-resistance of power transistors, which are sensitive to temperature changes and not suitable for high-current applications.
Innovation Solution
A current control circuit that includes a sense transistor coupled to a power transistor, with a control circuit to mirror the current flowing through the power transistor, generating a sense current and a control signal for controlling the power transistor's operation based on the sense current and a reference current, using a comparison circuit and an auxiliary voltage generating circuit to ensure bidirectional reference current and minimize on-resistance variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If resistance sensing is used for current sensing, then current detection can be achieved, but additional conduction losses are introduced
Solution Approach 1:
The patent introduces a sense transistor as an intermediary device to sense the current through its on-resistance. Instead of placing a separate sensing resistor in the power path, the sense transistor's channel resistance is utilized to generate a voltage proportional to the current, thereby eliminating the need for additional conduction losses while maintaining current sensing capability
Solution Approach 2:
The patent replaces the traditional mechanical/electrical resistance sensing method with a transistor-based sensing mechanism. By using the sense transistor's on-resistance characteristics and controlling its gate voltage, the system achieves current sensing without the energy losses associated with dedicated sensing resistors in the power path
2Ease of operation
If on-resistance of power transistor is used for current sensing, then current detection is possible, but accuracy deteriorates due to temperature variations
Solution Approach 1:
The sense transistor acts as an intermediary with a controlled on-resistance that is proportional to the power transistor's on-resistance. By controlling the gate voltage of the sense transistor, its resistance remains stable and proportional to the power transistor's resistance across temperature variations, providing accurate current sensing without being affected by temperature drift
Solution Approach 2:
The patent changes the operating parameters of the sense transistor, specifically controlling its gate voltage to maintain a proportional relationship between the sense transistor's on-resistance and the power transistor's on-resistance. This parameter control ensures that the sensing ratio remains constant despite temperature changes, improving measurement accuracy
3Productivity
If conventional current sensing is used, then current control can be implemented, but reliability decreases for high-current applications
Solution Approach 1:
The patent segments the power transistor function into two parts: the power transistor handles the high-current switching function, while the sense transistor handles the current sensing function. This segmentation allows the sensing function to be performed by a smaller, more reliable transistor that is not subjected to the same high-stress conditions as the power transistor, improving overall system reliability for high-current applications
Data Source
AI summary
A current control circuit for a power converter can include: a sense transistor coupled to a power transistor to be sensed; and a control circuit configured to control a voltage at a power terminal of the sense transistor in order to mirror a current flowing through the power transistor, to control the sense transistor to generate a sense current, and to generate a control signal for controlling operation states of the power transistor in accordance with the sense current and a reference current.